Acute kidney injury (previously known as acute renal failure) has a high morbidity and mortality. After developing a novel model of sepsis-induced AKI that employs cecal ligation puncture in elderly mice treated with fluids and antibiotics, we are using the model to study the pathophysiology of injury, to screen drugs, and to study their mechanisms of action, including conscious blood pressure by telemetry. We adjusted our mouse model by using outbred mice, which develop AKI at a younger age, and we established another model using comorbidity, namely pre-existing renal dysfunction, which is thought to increase susceptibility to AKI in patients. This acute-on-chronic syndrome has not been studied in animals, and the nephrology field is trying to gain more information about how this is manifest in patients. Because the model we used for pre-existing renal dysfunction is reversible, unlike the progression seen in CKD patients, we started with a partial renal ablation (5/6 nephrectomy) procedure, a classic rat CKD model, then adapted it to the mouse. We have characterized our model, and it has several hallmarks of progressive CKD, including hypertension, proteinuria, glomerulosclerosis, interstitial renal tubular fibrosis, anemia, and cardiac fibrosis. In order to make our CKD mouse model compatible with our sepsis AKI models, we tested three mouse strains, which had differential susceptibility to CKD. In the most susceptible strain, all aspects of CKD could be lowered by an angiotensin receptor blocker (olmesartan). Conversely, angiotensin II could convert a resistant strain to a susceptible strain. However, this effect is largely independent of blood pressure. 1) Previous studies had shown that calpains participate in a wide array of cellular functions that are associated with the severity of sepsis, including NFkB-mediated inflammation (cytokines, adhesion molecules, chemotaxis), apoptosis, and microparticle release. We examined the role of calpains in sepsis/sepsis-AKI using transgenic mice that ubiquitously overexpress calpastatin, an endogenous inhibitor of activated calpains. Calpastatin transgenic mice were protected from sepsis, and calpain inhibition prevented sepsis-induced microparticle release. When we added (reconstituted) microparticles from septic mice into septic transgenic mice; susceptibility to sepsis was restored. Microparticles transferred the disease, implicating microparticles as a prime target for calpain-mediated sepsis damage. Interestingly, spleen apoptosis and NF&#954;B/cytokines were not affected by this calpain/microparticle pathway, but disseminated intravascular coagulation (DIC) was decreased by calpain inhibition and worsened (restored) by microparticle reconstitution. This study suggests that targeting calpain may be a novel strategy to treat sepsis patients with diffuse intravascular coagulation (DIC), a marker for a highly aggressive form of sepsis. 2) We explored a counterintuitive role for phagocytosis that can promote sepsis, through a specialized form of bacterial entry into leukocytes. Certain intracellular bacteria (Listeria monocytogenes) can enter cells via lipid scavenger receptors SR-BI/II and CD36; inside the cell the bacteria enter a special niche, protected from lysosomal degradation (15). We tested whether this pathway was important in polymicrobial sepsis. SR-BI/II and CD36 knockout mice had improved survival, decreased inflammation, and decreased multiple organ damage including AKI. Unlike all our other studies, improved survival was associated with an increase in peritoneal neutrophils and monocytes. We demonstrated in vitro that granulocytes from these knockout mice had decreased intracellular bacterial invasion with a commensurate decrease in IL-6 release. A synthetic peptide that inhibits both scavenger receptors reduced mortality and sepsis-AKI. We propose that scavenger receptors allow intracellular bacterial invasion of phagocytes that triggers inflammation. The bacteria escape lysosomal degradation, proliferate intracellularly, and kill phagocytes. Scavenger receptor-deficient cells provide a new therapeutic paradigm by which bacteria can be killed more effectively without antibiotics (and associated antibiotic resistance) with less inflammation-induced organ damage. 3) In a follow-up study, these class B scavenger receptors recognize individual bacterial components such as GroEL and LPS, which can directly trigger pro-inflammatory cytokine release from phagocytes. Surprisingly, about half of the Gro-EL-stimulated cytokine response in vivo was mediated by these scavenger receptors. We continue to explore potential mechanisms and treatments for sepsis-AKI, CKD, and acute-on-chronic kidney disease.